The in vivo and in vitro properties of the transforming protein, p21 coded for by various members of the ras gene family have been investigated. The p21 for by v-ras-H has been purified to near homogeneity. The protein is a quanine nucleotide binding protein and autophosphorylates itself using GTP. The p21 coded for by c-ras-H, a normal cellular protoncogene has also been purified and found to be a quanine nucleotide binding protein. However, p21 coded for by c-ras-H-1 does not autophosphorylate. Thus, a major qualitative difference exists between the p21 of v-ras-H and the p21 of c-ras. The p21 of another ras gene containing virus, Balb-MuSV, is similar to the p21 of c-ras-H-1.